1. Field of the Invention
The present invention relates in general to a marine propulsion system, and more particularly to a transmission for a propulsion system of an outboard drive.
2. Description of Related Art
Many outboard drives of a marine watercrafts employ forward/neutral/reverse transmissions together with a dual propeller propulsion system. Such transmissions are common in both outboard motors and in outboard drive units of inboard/outboard motors.
These transmissions typically include a driving bevel gear and a pair of oppositely rotating driven bevel gears. Each driven gear includes a hub that is journaled within a lower unit of the outboard drive. A front dog clutch of a dual clutch assembly is interposed between the pair of oppositely rotating gears. In this position, the front dog clutch moves between positions in which the clutch engages the gears. The front dog clutch selectively couples an inner propeller shaft to one of the driven gears to rotate a first propeller in either a forward or a reverse direction.
The transmission also includes a second dog clutch that is positioned to the rear side of the rear driven gear hub. The rear clutch selectively engages corresponding teeth formed on the rear side of the hub of the rear gear to drive an outer propeller shaft.
Such prior transmission designs tend to occupy a significant amount of space in the lower unit which also houses an exhaust passageway for the discharge of engine exhaust. Outboard drives commonly discharge engine exhaust beneath the water level of the body of water in which the outboard drive is operated in order to silence exhaust noise. For this purpose, the exhaust system passes through the lower unit and the propulsion device, and discharges behind the propulsion device at a lower pressure region produced by the propulsion device.
The large size of prior transmission used with counter-rotational propulsion systems commonly leaves less than adequate space for the exhaust passage though the lower unit. Higher back pressure results, and the exhaust tends not to discharge smoothly. Engine performance consequently suffers.
With the direction in the marine industry towards larger engines (i.e., larger cylinder bores or increased number of cylinders), this problem becomes more acute. It becomes necessary to increase the flow area of the exhaust passage through the lower unit in order to discharge exhaust gas smoothly. The lower units thus have increased in size to accommodate the larger exhaust passages with the current transmission designs. An increased size in the lower unit, however, undesirably increases the resistance to fluid flow around the lower unit, i.e., undesirably increases the drag on the lower unit.